The present invention relates to a temperature sensing circuit, and more particularly, to a technology for reducing the size of a temperature sensing circuit and detecting temperature with a high accuracy.
A temperature sensing circuit in a semiconductor memory device refers to a circuit applied on a semiconductor chip to measure temperature. Especially, the temperature sensing circuit is generally used in a conventional Dynamic Random Access Memory (DRAM), and it will be explained how the temperature sensing circuit is applied to the DRAM.
A cell of the DRAM includes a transistor acting as a switch and a capacitor for storing electric charges (data). Data of the cell is distinguishable between “high” (logic 1) or “low” (logic 0) according to whether electric charges exist in a capacitor of a memory cell, that is, whether a terminal voltage of a capacitor is high or low.
Since data is stored in form of electric charges accumulated in a capacitor, there should be no power consumption ideally. However, leakage current caused through P-N junction of a transistor may exist and discharge initially stored electric charges. That is, the stored data may be lost. To prevent this problem, data in a memory cell is read before being lost and the normal level of electric charges is restored according to the read information.
This operation is repeated periodically to retain data and this cell recharging process is called a refresh operation. Typically, the refresh operation is controlled by a DRAM controller. In performing the refresh operation and the control thereof, additional power is consumed in the DRAM.
A concern in a battery-operated system with lower power is the reduction of power consumption. An example of an attempt to reduce the power consumption necessary for the refresh operation is to change a refresh period according to temperature. More specifically, data retention time in the DRAM becomes longer as temperature becomes lower. Accordingly, when a temperature range is divided into a plurality of regions and a frequency of a refresh clock is decreased to be relatively low in a low temperature region (the refresh period is increased), it is apparent that power consumption will be decreased. Therefore, a temperature sensing circuit which can accurately detect temperature in the DRAM and output information about the detected temperature may be useful.
Moreover, as the integration level and operating speed of the DRAM are increased, a large amount of heat is generated in the DRAM itself. The increased temperature of the DRAM due to the generated heat may cause the interruption of the normal operation and the failure of the DRAM itself. Accordingly, there is a growing need for a temperature sensing circuit which can accurately detect the temperature of the DRAM and output information about the detected temperature.
FIG. 1 is a block diagram illustrating the structure of a typical temperature sensing circuit.
The temperature sensing circuit includes a temperature-dependent voltage generator 110 for outputting a temperature-dependent voltage VTEMP varying with temperature, and an analog-to-digital converter 120 for converting the temperature-dependent voltage VTEMP into a digital thermal code THERMAL_CODE.
Specifically, the temperature-dependent voltage generator 110 detects temperature based on the fact that a change of a base-emitter voltage (VBE) of a bipolar junction transistor (BJT) is about −1.8 mV/° C. in a bandgap circuit generating a bandgap voltage that is not affected by temperature and supply voltage. Moreover, the temperature-dependent voltage generator 110 amplifies the slightly-changing base-emitter voltage (VBE) of the BJT to output the temperature-dependent voltage VTEMP that is in 1:1 correspondence to the temperature. That is, the temperature-dependent voltage generator 110 generates the temperature-dependent voltage VTEMP that decreases as temperature increases.
The analog-to-digital converter 120 converts the temperature-dependent voltage VTEMP outputted from the temperature-dependent voltage generator 110 into the digital thermal code THERMAL_CODE. The analog-to-digital converter 120 is typically implemented with a tracking analog-to-digital converter.
FIG. 2 is a circuit diagram illustrating the structure of the temperature-dependent voltage generator 110 of FIG. 1.
The temperature-dependent voltage generator 110 is a kind of a bandgap circuit. In FIG. 2, both of a unit for generating a temperature-dependent voltage VTEMP and a unit for generating a reference voltage VREF are shown. Although only the unit for generating the temperature-dependent voltage VTEMP is related to the temperature sensing circuit, the typical bandgap circuit is designed to generate both the temperature-dependent voltage VTEMP and the reference voltage VREF.
First, the generation of the temperature-dependent voltage VTEMP will be described.
A voltage VBE2 of a BJT Q2 is inputted into an operational amplifier 101, and voltages of two input terminals (+, −) of the operational amplifier 101 become equal due to a virtual short principle. That is, VTEMP=(1+R10/R9)*VBE2. Since the base-emitter voltage VBE2 of the BJT Q2 is changed according to temperature, the temperature-dependent voltage VTEMP that is the amplified base-emitted voltage VBE2 is also changed according to temperature.
The generation of the reference voltage VREF will be described.
Emitter currents of two BJTs Q1 and Q2 having a ratio of N:1 are expressed as the following equation.IQ1=IS*exp[VBE1/VT], IQ2=N*IS*exp[VBE2/VT]where VT is a temperature coefficient and IS is a saturation current.
When electric potentials of VBE1 and X node are equal by the operational amplifier 102, a current IPTAT flowing through a resistor R1 is expressed as the following equation.IPTAT=(VBE1−VBE2)/R1=ln(N*A)*VT/R1
Moreover, under the same condition, a current ICTAT flowing through resistance R2 is expressed as the following equation.ICTAT=VBE1/R2
That is, the current IPTAT increases in proportion to temperature, and the current ICTAT increases in inverse proportion to temperature.
Under the assumption that the same amount of current flows through the MOS transistors having the same size, currents M*IPTAT and K*ICTAT become M*IPTAT and K*ICTAT as indicated.
The reference voltage VREF outputted based on the above fact is expressed as the following equation.VREF=K*R3/R2*(VBE1+(M*R3)/(K*R1)*ln(N*A)*VT)
When values of M, R1, R2, R3, K and M are appropriately adjusted in order to compensate for temperature, the reference voltage VREF has a constant value at all times regardless of temperature change. Generally, while the values of N, R1, R2 and R3 are fixed, only the values of K and M are adjusted in order to allow the reference voltage VREF to have a constant value at all times regardless of temperature.
Since the generated reference voltage VREF is constant even when temperate is changed, it is used as a reference voltage in various internal circuits of a chip.
As mentioned above, the temperature-dependent voltage VTEMP is expressed as VTEMP=(1+R10/R9)*VBE2. Since the value of VBE2 is changed according to temperature, the temperature-dependent voltage is also changed according to temperature.
However, the value of VBE2 is not changed only by the change of temperature. The value of VBE2 is also affected by process variations. When the value of VBE2 is changed by the process variations of the BJT, the temperature-dependent voltage is also changed.
Therefore, when the temperature sensing circuit outputting the thermal code by using the temperature-dependent voltage VTEMP is affected by the process variations, errors may occur in the measured temperature whenever processes are changed.
FIG. 3 is a block diagram illustrating the structure of the analog-to-digital converter 120 of FIG. 1.
The analog-to-digital converter 120 includes a voltage comparing unit 310, a counting unit 320, and a converting unit 330. The analog-to-digital converter 120 converts the temperature-dependent voltage VTEMP outputted from the temperature-dependent voltage generator 110 of FIG. 1 into the digital thermal code THERMAL_CODE.
In operating the analog-to-digital converter 120, the converting unit 330 is a digital-to-analog converter which outputs an analog voltage DACOUT in response to the digital thermal code THERMAL_CODE outputted from the counting unit 320. The voltage comparing unit 310 compares the temperature-dependent voltage VTEMP with the analog voltage DACOUT. When the temperature-dependent voltage VTEMP is lower than the analog voltage DACOUT, the voltage comparing unit 310 outputs a decrement signal DEC for decreasing the value of the digital thermal code THERMAL_CODE by the counting unit 320. On the other hand, when the temperature-dependent voltage VTEMP is higher than the analog voltage DACOUT, the voltage comparing unit 310 outputs an increment signal INC for increasing the value of the digital thermal code THERMAL_CODE by the counting unit 320. The counting unit 320 receives the increment signal INC or the decrement signal DEC from the voltage comparing unit 310, and increases or decreases the digital thermal code THERMAL_CODE stored therein.
In summary, the analog-to-digital converter 120 repeatedly performs the operation of comparing the temperature-dependent voltage VTEMP with the analog voltage DACOUT to increase or decrease the digital thermal code THERMAL_CODE. Thus, the analog voltage DACOUT tracks the temperature-dependent voltage VTEMP, and the digital thermal code THERMAL_CODE obtained when the tracking is completed becomes a digital value that is converted from the temperature-dependent voltage VTEMP.
However, since the tracking analog-to-digital converter includes the complicated logic circuits such as the voltage comparing unit 310, the counting unit 320, and the converting unit 330, it occupies a relatively large circuit area.